José Nunes Cordeiro

Hydrogels are three-dimensional polymer matrices with recognized biomedical applications. Chemically crosslinked hydrogels offer greater mechanical stability than physically crosslinked hydrogels due to the covalent bonds between their polymeric chains. The preparation of hydrogels by chemical crosslinking involves three basic components: monomer, initiator and crosslinking agent, which must be present in proportions that do not alter the integrity of the hydrogel. The chemical crosslinking mechanism can be designed from reactions between complementary functional groups, ultraviolet light reactions, radical polymerization, high energy irradiation, among others. In this review, we revisit the chemical crosslinking mechanisms involving synthetic or natural polymers. Finally, biomedical applications of hydrogels are discussed, such as drug delivery, cell culture, tissue engineering, cancer therapy, among others.

Hydrogels are three-dimensional networks formulated from natural or synthetic polymers with a high capacity to absorb and transport water in their structure. Hydrogels are prepared from the crosslinking of their polymeric chains, which involves two basic mechanisms: chemical crosslinking and physical crosslinking. In chemical crosslinking, hydrogels are held together by covalent bonds; while physically cross-linked hydrogels are produced by non-covalent interactions, such as hydrogen bonds, electrostatic interactions, and hydrophobic forces, among others. Physically cross-linked hydrogels are more similar to biological systems due to their assembly dynamics, so they have wide biomedical applications. The most used approaches in the preparation of hydrogels by physical crosslinking include freeze-thaw, formation of stereocomplexes, ionic interaction, hydrogen bonding, crystallization, and crosslinking by hydrophobic interactions. These approaches are briefly discussed in this review. Some biomedical applications of these hydrogels will also be discussed.